Trouble with old amp: voltages too low. Bad power transformer?

[slidincharlie (Carlo P)]

I have a friend's Silvertone 1482 amp on my bench. Schematic is in attachment.
All the tubes and the pilot lamp lit, but no sound comes from the speaker. If I touch the jack legs inside the chassis with my bare fingers I hear a sort of very weak buzz coming from inside the chassis, not from the speaker.
The rectifier (a fresh NOS 6AX4) shows only 255 VAC on the plates and 266 VDC on the cathode. As a consequence all voltages are lower than specs throughout the amp (I checked only the filter caps nodes and the plates on the power tubes actually)
The heaters read 6.16 VAC between both ends.
Readings don't change with another new 6AX4.

Could it be the power transformer?
What readings should I expect if I disconnect its wires?

With such low tension going to the amp, shouldn't the amp sound all the same (maybe weaker than normal)?

[Don Moose]

I'd actually think about leaky/declining filter caps before a faulty PT. If original, they're freaking ancient and are likely to have lost capacitance.

[slidincharlie (Carlo P)]

Don,
do you mean that faulty filter caps can cause such a drop in all voltages and no sound from the speaker?
Of course fresh electrolytic caps are the first job I'm gonna do in this amp. But first I'd like to face the low voltages issue.

[Don Moose]

Good caps are essential to building up that DC voltage - as the caps charge, the rectifier is going to move through providing a 120Hz half-sine (a sine slit along the X axis, with the negative peaks reflected to positive) that will decrease in amplitude as the DC voltage it's riding on builds.

If the caps are too low a value to sustain the DC between peaks, or too leaky to maintain a charge, that's not going to happen and your meter, set to DC is going to read low. What reading do you get on an AC voltage setting?

Voltages can also be lowered by excessive loads - if you remove the output tubes, what happens to the B+ voltage? - what about if you disconnect the B+ rail to the preamp?

With the rectifier removed, what AC voltage do you get at the anode pins?

This should help isolate the fault to either the power supply (filter or PT at this point) or load (power amp or preamp).

Do you have the correct voltage going INTO the PT? might there be some element between the wall and the PT that's keeping you down?

Hope this helps!

[slidincharlie (Carlo P)]

New readings

Don,
the good news is that the amp was dumb just because the speaker was dead! I tried a good replacement speaker and now the amp makes sound.

Second: voltage readings.
I'll make you a summary of my readings:
253 VAC at the rectifier's anodes;
257 VDC at the rectifier cathode;
256, 237 and 210 VDC at the three filter cap nodes;
79 and 76 VDC on the first preamp anodes;
143 and 120 VDC at the second preamp anodes;
246 and 255 VDC at the power tubes plates.

If you read the schematic attached to my first post, these readings are much lower than expected.
The above readings do not change with different rectifier and different power
tubes.

Now here's what happens with the trials that you suggested:
- with power tubes removed the B+ increases from 257 to 358VDC on the rectifier cathode. On the filter caps it increases from 256 to 358VDC on the 20uF, from 237 to 349VDC on the 10uF, and from 210 to 304VDV on the 5uF.
- with the rectifier removed and the power tubes in place, I get 265VAC at its anode pins.
- with the B+ wire lifted from the rectifier's cathode and power tubes in place, the recto anodes are still 265VAC and the cathode reads a very low 240VDC.

What do these readings suggest to you?

[Enzo]

With the rectifier removed, the high voltage AC goes no further than the rectifier socket, so it doesn't matter whether the other tubes are in there or not - there will be no B+ at all.

If you put the rectifier tube in and disconnect the B+ wire from it, then it has no filter cap, and your raw pulsing DC will of course be very low.


With the 257 AC you report, we might expect 350 volts B+ or so, and without the power tubes it gives you exactly that. Under load it will drop some, but not 100 volts.

Obviously your tubes are loading this power supply down.

Two ways - among many - this can happen. One is simply that the tubes are defective - at least one of them. The other is that the filter caps are so dried out they offer no ability to hold the B+ up under load.

When you measured the very low DC of 257DC at the first filter, you should flip your meter over to AC volts. This will measure the ripple voltage across the cap. See if that goes up to 50-100 volts AC of ripple.

IN any case, plug a different pair of 6V6s in there. ANy difference? Or at least remove one of the existing ones and see what hap-pens. Then try it with the other one. You might find one tube is bad instead of two. If either one loads it down, then either both are bad, or the power supply can't hold up. If you are still not sure take one 6L6 and plug it into one socket and see what the B+ does. Now move it to the other socket. The 6L6 is not right for the amp, but it is close enough to see if the power supply dies no matter what.

And look at this - what voltage is on the cathodes of the 6V6s when this is on? A shorted cathode cap would remove all teh bias and the power tubes would crank up the current. They would load down the B+ but they also would get pretty hot.

And check this - with the power tubes out, what voltage appears at their grid pins on the sockets pin 5? SHould be zero, but if positive voltage appears, one of the coupling caps is leaky - those o.o1uf caps from the phase splitter.

And when you turn down the trem intensity all the way, is point A between the power tube grid resistors really grounded?

But really, I think you need some new caps.

[Bruce / Mission Amps]

I think it is simply bad power tubes.... soft or under biased and pulling way to much DC input power.
Enzo's comment about a bad power tube bypass cap is good too.

[slidincharlie (Carlo P)]

Enzo, Bruce, Don:
I believe that this amp suffers a mixture of the pains that you have enumerated.

I have done all the trials suggested by Enzo.

The power tubes maybe are a little bit weak, but they do not seem the main culprit. Pulling either tube does not affect much the B+, it only increases a few volts. Same readings after putting another pair of good 6V6's, too.
But... they are biased hot: they run at about 31 mA each. I checked the cathode cap and bypass resistor: for some reason, even though they looked stock, they were off specs (10uF@25V and 220 ohm, instead of 25uF@25V and 270 ohm as per the schematic). I replaced them with the correct values, but the tubes idle still around 29 mA.
Anyway with new cathode cap and bypass resistor the B+ is a little higher, around 274 V (was 257 V).
With a new pair of NOS 6V6 things get better: 260 VAC at the rectifier plates, B+ at 283 VDC, bias at 27 mA. Lower than specs still.

The ripple across the first filter cap is around 9 VAC.

The readings on pin 5 of each power tube is .08 and .14 VDC. Too low to consider those coupling caps leaky, right?

Point A of the tremolo circuit, with the amp shut off and the intensity pot at 0, reads about 80 ohms; it is not a steady reading, it oscillates somewhat. Should it be zero ohm?

I think I covered all of Enzo's points.

Maybe new filter caps (which I do not have at hand now) will cure everything?

What do these findings suggest?

[cbarrow7625]

9VAC ripple??? That's a bit excessive!

I'm coming in late here but all the evidence seems to point to poor filter caps at this point. Since you have tried new tubes & replaced the power tube cathode resistor & cap, that is about all that is left.

If the amp never had them replaced it is probably about time anyway.

Pull the old filter caps and see if you can measure a dc resistance across them...you shouldn't be able to but I'll bet you can with yours. That is what happens when the capacitor goes bad & the DCR goes way up. Unwind the foil laers in the cap & you may even find foil layers shorting all the way through the cap.

I remember reading a long time ago about how it was possible for a filter cap to have electrons "let go" inside them & actually tear through the cap the way a piece of space dust could rip through a space craft. It was in a discussion around "what happens if the amp runs with no load" (vs. the shorted output that most tube amps have with no load) and one of the ideas that was discussed was this one of the effects that could have on the filter caps if the amp were run that way at high levels.

I wish I could remember more of that discussion in context. does that ring a bell for anyone else here?

Chris

[slidincharlie (Carlo P)]

Chris,
thanks for your comment.
Changing filter caps is always the first thing I do in any old amp, and I have already ordered them. This is the first time that I observe a malfunctioning due to old filters, though. Interesting experience...

[Don Moose]

I don't have a _good_ reason for thinking this, but have you looked at the screen resistor and voltage?

Once the filter cap issue is settled, if you still have problems, I think I'd look there next.

Enzo - would a screen resistor that's too high (or is it low) make things run a bit hot?

[Enzo]

I doubt it, but it would take mere seconds to measure it. SO why not lets do that? If it opened or went really high, the tube would not want to conduct. Turning off the screens is a method of putting an amp in standby for some OEMs.

Most amps have enough power supply balls to red plate their tubes without loading the B+ down 100 volts don't they, so if the screens went nuts somehow it still ought not to cause his symptom.

[Don Moose]

Most amps have enough power supply balls to red plate their tubes without loading the B+ down 100 volts don't they, so if the screens went nuts somehow it still ought not to cause his symptom.

That was the bit I needed - thanks.

So, I'm back to 1) decaying filters or 2) unexpected impedance in the PSU - maybe the PT's secondary's CT?

Hope this helps!

[slidincharlie (Carlo P)]

A few new readings...

1) after cleaning the tremolo pot, point A between the screen resistors and the tremolo footswitch jack reads around 15 ohm, which is exactly the value of the trem pot turned full down. So I guess this is ok, isn't it?

2) The 330k screen resistors read 310k and 350k respectively. The screen voltage at V3 (310k) reads about .06 VDC, while at V4 (350k) reads around 9 VDC. The above values rise to 15 and 20 VDC respectively when I turn the trem pot full up. With two new screen resistors the screen voltages are a tiny little bit less unbalanced (around .6 and 6 VDC on V3 and V4 respectively).
No effect on the B+ or plate voltage.

What does this unbalance on the screen voltages mean?

[Enzo]

What are you calling screens? Pin 4 of the power tube is the screen, and it should have B+ voltage on it. Pin 5 is the control grid. It should have more or less zero DC on it. If it is sitting there at 6 or 9 volts DC, then the 0.01 cap from the phase splitter is leaking bad. The +9v will cause the tube to conduct hard, and that will drag the B+ down.